粤北白沙岩体早白垩世细粒花岗岩年代学、地球化学特征及成因意义

Geochronology,Geochemistry and Genetic Significance of Early Cretaceous Fine Granite in Baisha Rock Body,Northern Guangdong

粤北佛冈岩体北缘的白沙岩体细粒黑云母二长花岗岩LA-ICP-MS锆石U-Pb加权平均年龄为130.3±3.0Ma,表明细粒黑云母二长花岗岩侵入于早白垩世。岩体高硅(SiO2=70.64~75.01wt%),高Na2O+K2O(7.62~8.33wt%),低P2O5(0.024~0.100wt%),富集Rb、Th、U、K、Pb、Nd、Zr和Hf等元素,亏损Ba、Nb、Ta、La、Ce、Sr、P和Ti等。明显富集轻稀土元素[(La/Yb)N=6.74~54.69],显示明显的负Eu异常(δEu=0.15~0.35),少量正Eu异常(δEu=0.82~1.19)。这些地球化学特征指示细粒黑云母二长花岗岩为高分异I型花岗岩。锆石Hf同位素研究结果表明,εHf(t)=-7.3^-10.6,其平均值为-8.89。其两阶段模式年龄(T2DM)主要为1654~1865Ma(n=8),平均值为1751Ma。佛冈复式岩体早白垩世细粒黑云母二长花岗岩形成于古太平洋板块向欧亚板块俯冲的陆缘弧构造背景下,其地球化学特征表明其物质主要来源于古元古代地壳的杂砂-泥质源岩部分熔融,并有少量幔源物质参与。

The Baisha granite located in the northern edge of the Fogang rock mass, its main rock being the fine-grained granite. The weighted average ages of the zircon ages is 130.3±3.0 Ma, indicating that the fine-grained biotite monzonitic granite intruded in the Early Cretaceous. Baisha granite is high in silicon (SiO2=70.64-75.01 wt%), high in Na2O+K2O (7.62-8.33 wt%) and low in P2O5 (0.024-0.100 wt%). It is rich in Rb, Th, U, K, Pb, Nd, Zr and Hf, and depleteed in Ba, Nb, Ta, La, Ce, Sr, P and Ti. The light rare earth elements [(La/Yb)N=6.74-54.69], show obvious negative Eu anomalies (δEu=0.15-0.35), and a few positive Eu anomalies (δEu=0.82-1.19). These geochemical characteristics indicate that fine-grained biotite monzonite is highly differentiated I-type granite. The results of zircon Hf isotope study show that of εHf(t)=-7.3 ~ 10.6, the average value is -8.89. The two-stage model age (T2DM) ranged from 1 654 to 1 865 Ma (n = 8), with an average of 1 751 Ma. The early Cretaceous fine-grained biotite monzonitic granite of Fogang complex intrusion was formed in the continental margin arc tectonic background of the subduction of the Paleo-Pacific plate to the Eurasian plate. Its geochemical characteristics indicate that its material mainly comes from the partial melting of the mixed sand-mudstone source rocks of the Paleoproterozoic crust, with a small amount of mantle-derived material participating.